[ruby-core:75462] [Ruby trunk Bug#12373] Optimize CSV#shift

Issue #12373 has been reported by Yuki Kurihara.

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Bug #12373: Optimize CSV#shift
https://bugs.ruby-lang.org/issues/12373

* Author: Yuki Kurihara
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.4.0dev (2016-05-11 trunk 54976) [x86_64-darwin15]
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
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I think that **str1.start_with?(str2)** is faster than **str1[0] == str2**.
Because **str1.start_with?(str2)** just call **String#start_with?**, But **str1[0] == str2** make string object, call **String#[]** and call **String#==**.

(The patch is csv-shift.patch)

Benchmark results.

csv-benchmark.rb make temp CSV file and call **CSV#each** method(inner call **CSV#shift**)

~~~
$ ruby csv-benchmark.rb
Warming up --------------------------------------
       old_csv_shift     1.000  i/100ms
       new_csv_shift     1.000  i/100ms
Calculating -------------------------------------
       old_csv_shift      0.444  (賊 0.0%) i/s -      3.000  in   6.759200s
       new_csv_shift      0.479  (賊 0.0%) i/s -      3.000  in   6.264069s

Comparison:
       new_csv_shift:        0.5 i/s
       old_csv_shift:        0.4 i/s - 1.08x slower
~~~

string-start_with.rb is a micro benchmark for **str1[0] == str2** and **str1.start_with?(str2)**

~~~
$ ruby string-start_with.rb
Warming up --------------------------------------
           a[0] == b    90.881k i/100ms
    a.start_with?(b)   115.557k i/100ms
Calculating -------------------------------------
           a[0] == b      1.836M (賊 3.8%) i/s -      9.179M in   5.006568s
    a.start_with?(b)      3.183M (賊 4.2%) i/s -     15.947M in   5.018654s

Comparison:
    a.start_with?(b):  3183386.0 i/s
           a[0] == b:  1836263.5 i/s - 1.73x slower
~~~

Of course $ make test-all TESTS="test/csv/*" passed

---Files--------------------------------
csv-benchmark.rb (561 Bytes)
old_csv_shift.rb (5.1 KB)
new_csv_shift.rb (5.11 KB)
string-start_with.rb (215 Bytes)
csv-shift.patch (434 Bytes)


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